Optimal laser scanning path for larger structures case hardness uniformity of steels

When larger-sized industrial parts need to be locally hardened by a laser, due to laser focal spot limitation, number of laser scanning is essential to cover the larger surface. In this case, optimal scanning path is very crucial to obtain case hardness uniformity.  The case hardness uniformity is dependent on the tempering temperature generated during the successive scans of laser beam over the previously heat-treated area. Therefore, case hardness uniformity is a function [f(HV) = f(Ttmp)] of the tempering temperature. The tempering temperature is a function [f (Ttmp) = f (Haccumulation)] of heat accumulation. The heat accumulation is a function [f (Haccumulation) = f (Ov, Ls)] of size of overlap and length of scan for a constant laser energy density [LED=P/A×(l or d)/v ], where P, laser power, A, laser focal spot area, v scanning speed, l or d, dimension along scanning path.      For larger structural steels, the following three cases were seen using a finite element modeling approach by varying the size of overlap and length of scan for a constant laser energy density (refer below figure).

Case 1: (Ttmp > AC1). For a bigger size of overlap and shorter length of scan, the tempering temperature is above the AC1 temperature. A higher heating and reaustenization have occurred. A portion of austenite and martensite mixture has been obtained.

Case 2: (AC1 < Ttmp < AC3). For a medium size of overlap and length of scan, tempering temperature falls between the AC1 and AC3 temperatures. A full austenization occurs in the heat treated material; then by rapid cooling, a complete martensite transformation was achieved.

Case 3: (Ttmp < AC1). For a smaller size of overlap and longer length of scan, tempering temperature falls below the AC1 temperature. A partial austenization occurs; then by controlling the cooling cycle, a mixture of phases (bainite, ferrite, and pearlite) was obtained.